Elevator systems are useful for carrying passengers among different levels in a building. Typical traction-based elevator systems include elevator cars and counterweights associated with respective machines responsible for moving the elevator car. Some elevator machines are capable of operating in two different modes. In a motoring or power consumption mode, the machine draws power from a utility grid or emergency generators, for example, while starting movement of the elevator car or lifting a positive load. In a power regeneration or “regen” mode, the machine operates as an electrical generator generating electricity that can be provided back into the utility grid, emergency generators or an energy storage device. The regeneration mode may occur, for example, when stopping a moving car or lifting a negative load based on movement of the elevator car under appropriate conditions.
Many elevator systems include a backup power source to allow elevator system operation even when a primary power supply becomes unavailable, such as during a utility power outage. The amount of power drawn by a typical elevator system requires a substantial backup power supply. Many existing elevator systems include a restriction or limitation on the number of elevator cars that can be in service when the backup power supply is in use. For example, some elevator systems will only allow one car to be in service under those conditions. The Occupant Evacuation Operation (OEO) protocol requires sufficient back up power to supply all the Occupant Evacuation Elevators (OEEs) in a building. One approach to satisfying OEO requirements would be to include multiple, large-capacity emergency generators, but that introduces significant cost.